a) Field of the Invention
The present invention relates to ultrasonic monitors for measuring heart and pulse rates in living subjects. Methods for measuring heart and pulse rates of living subjects through ultrasonic means are also encompassed by the instant invention.
b) Description of Related Art
Measuring Heart and Pulse Rates
Measuring heart and pulse rates in living subjects has been accomplished by various means. The pulse rate is commonly measured by lightly touching one's fingers over an artery and counting the rate of pulsation. The heart rate is usually measured by a sensing device using electrodes that monitor the electrical activity of the heart (e.g., contact monitors) based on electrocardiograms (EKG OR ECG). Measuring rate values is a useful tool in individualizing and optimizing exercise regimens. Individuals who want to increase endurance or performance aim for certain target heart rates to maximize progression towards their goals. Conversely, adults with a history of heart disease must avoid exceeding a certain heart or pulse rate to reduce unnecessary strain on the heart and resultant injury.
The heart rate is the rate of contractions over a given time period, usually defined in beats per minute. A pulse can be defined as the rhythmical dilation of a vessel produced by the increased volume of blood forced into the vessel by the contraction of the heart. The pulse can be felt at many different points on the body, including the wrist (radial artery) and neck (carotid artery), which are among the most easily accessible points. Since a heart contraction almost always produces a volume of blood that can be measured as a pulse, the heart rate and pulse rate are usually the same. However, there are certain situations where the pulse rate may differ from the heart rate. For example, the body may generate an irregular heart beat or a premature heart beat. In this scenario, a heart contraction would not force out enough blood to be measured as a pulse and the measured pulse rate would be different from the heart rate.
Heart rate monitors that provide continuous heart rate readings rather than a single point measurement require wearing a chest strap. There are a few heart rate monitors that do not require a chest strap. Most, if not all, of these monitors do not provide continuous heart rate readings but measure the wearer's pulse and transmit that pulse upon request. Most users would have to stop exercising in order to get this type of measurement, which is disruptive to an exercise regimen. In U.S. Pat. Nos. 5,738,104 and 5,876,350 and European Patent No. 0861045B1, Lo et al disclosed an EKG heart rate monitor that does not require a chest strap so that the user does not have to stop exercising to take a heart rate measurement. All the sensors and electronics are contained in a wristwatch. The software is effective in filtering out muscle motion noise. Therefore the user can walk and jog while taking a single point measurement. However, this technology still does not offer continuous readings. Hence, most users or heart patients that demand continuous heart rate readings choose a monitor that requires a chest strap. Most of the population, including the elderly, would prefer a monitor that does not require a chest strap. There are also portable patient monitors (e.g., vital signs monitors, fetal monitors) that can perform functions as diverse as arrhythmia analysis, drug dose calculation ECG waveforms cascades, and others. However, such monitors are usually fairly large (e.g., size of a small TV) and are connected to the patient through specific wires. The art has, thus, a need for an improved heart monitoring device, specifically one that provides continuous heart rate readings for both healthy and compromised living subjects without the need for chest straps, wirings, or the like.
Since the advent of the wristwatch, the wrist has offered a convenient, accessible, and non-intrusive location for an individual to wear a mechanical device. Moreover, the shallow depth of the radial artery in the wrist offers a number of advantages for allowing the continuous detection of blood rate pulses. Many different sensor types for pulse detection in the wrist have previously been developed.
Im & Lessard, in “Proceedings of IEEE-EMBC & CMBEC”, 2:1033-1034 (1995) and Tamura et al., in “Proceedings of IEEE-EMBC & CMBEC”, 2:1591-1592 (1995) describe implementation. Pulse detection in heart rate measurement has been implemented by means of piezoelectric sensors where the mechanical stimulus generated by the pressure pulse is converted to an electrical signal for further signal processing.
Dupuis & Eugene, in “IEEE Transaction on Instrumentation & Measurement”, 49:498-502 (2000) describe use of a strain gauge differential pressure sensor in a measurement system, where a low pressure cuff was wrapped around the wrist and then the pressure modulation in the cuff caused by the pressure pulse was measured with strain gauges.
Sorvoja H., in her Licentiate Thesis, University of Oulu (1998—in Finnish) and Ruha et al., in Proceedings of Biosignal 1:198-200 (1996) describe utilization of new pressure sensitive materials like electromechanical film (EFMi) and polyvinylidene fluoride (PVDF) in sensors for pulse detection in the radial artery
Gagnadre et al., in Electronic Letters, 32:1991-1993 (1998) describes the use of fiber optic sensors to detect heart rate. A multimode optical fiber was placed between two aluminum plates. The force generated by the pressure pulse caused variation in the modal distribution in the fiber and the pulse is detected using a photodetector.
Infrared optical sensors in cardiovascular pulse detection typically measure the optical power variation which is due to absorption or scattering when the amount of blood in the measurement volume varies. This kind of measurement, known as photo-plethysmography (PPG), was first disclosed by Herztman, “Photoelectric Plethysmography of the fingers and toes in man”, Proceedings of the Society for Experimental Biology and Medicine 37:1622-1637 (1937).
PPG is mainly used for measuring pulsation in a capillary network. Workers such as Hast, “Optical heart rate detection structures & methods. Thesis for the Diploma Engineer Degree”, University of Oulo (Finnish), and Aritomo et al., “A wrist-mounted activity and pulse recording system”, Proc. of 1st Joint BMES/EMBS Conf. 2:693 (1999), have applied PPG to measurements above the radial artery.
Sensors that monitor pressure pulses in the wrist such as mentioned above suffer a common problem. The pressure pulses are generally attenuated by the tissues between the artery and the sensor such that much of the high frequency components in the signal are lost. When the subject is in motion, muscle movement may create substantial noise at the pressure sensors. These noise signals are low frequency in nature. They will thus make it very difficult to identify blood pressure pulses reliably. Photo-plethysmography (PPG) suffers similar problem that when the interface between the photo detector and the wrist is not stable due to motion, the intensity of the transmitted or reflected light signal may be significantly disturbed.
The ambient lighting condition also plays an important role to the effectiveness of PPG technology. The various different technologies using strain gauge, piezoelectric film material, infrared optical coupler pair and fiber optic sensor can only measure heart rate with reasonable reliability when the subject is still. They are not practical for sports, fitness and rehabilitation applications where the subject is moving.
It is well known in the prior art to employ sonar technology to identify moving objects. A piezoelectric crystal may be used both as the power generator and the signal detector. In this case, the ultrasonic energy is emitted in a pulsed mode. The reflected signal is picked up by the same crystal after the output power source is turned off. The time required to receive the reflected signal depends upon the distance between the source and the object. The frequency shift, better known as Doppler shift, is dependent upon the speed of the moving object. This technique requires only one crystal but the detector circuit will only work after the transmitter power is turned off. It is conceivable to use this method to detect the motion of a blood vessel wall to extract the pulse rate information. However, for superficial blood vessels this technique requires very high speed power switching due to the short distance between source and object. In addition, muscle movement will also generate reflections that compromise the signal-to-noise-ratio in the system. The muscle noise signal in this case is very similar to the signal due to blood vessel wall motion. Therefore, it is very difficult to detect heart rate this way when the living subject is in motion. The advantage of this approach, however, is low cost and low power consumption. For continuous mode two piezoelectric elements may be used. Either may be used as the transmitter and the other as receiver or detector at a given time. These two elements can be positioned at an angle to the direction of the flow on opposite sides or on the same side of the conduit. If they are on the same side, the two crystals can be conveniently packaged into a module. The flow rate or flow velocity is proportional to the Doppler shift relative to the operating frequency. The main advantage of continuous mode for pulse rate application is that the Doppler shift due to blood flow is distinctly different from the shifts due to muscle artifacts or tissue movement. The shift due to blood flow is higher in frequency than that due to muscle motion. Therefore, even if the muscle motion induced signals are larger in amplitude, they may still be filtered out by a high pass filter in either analog or digital form to retain the blood flow signals. In this respect the ultrasound method is superior to infrared, pressure sensing and even EKG based technologies.
One device useful for the measurement of heart and pulse rates is an electronic unit worn on the wrist. Several such devices are known in the art. U.S. Pat. No. 4,086,916 (Freeman et al.) discloses a cardiac wristwatch monitor having ultrasonic transducers mounted in the wrist strap portion. The transducers are encased in an epoxy and covered with an insulative coating. U.S. Pat. No. 4,163,447 (Orr) discloses a wrist-mounted heartbeat rate monitor that relies upon light-emitting diodes. U.S. Pat. No. 4,256,117 (Perica et al.) discloses a wrist-mounted combination stopwatch and cardiac monitor that uses a pressure transducer to measure pulse rate.
In Freeman's invention, a wristwatch was intended to offer a continuous pulse rate monitor. However, ultrasonic energy is prone to diffraction and attenuation at the interface of two media of different densities. Any air gap at the interface or any air bubbles in the media will also make ultrasonic energy transfer unreliable. Therefore, it has been a standard practice to apply water or an aqueous gel between the transducer module and the living subject to eliminate any air gap. Unfortunately water and aqueous gels dry up quickly in open air. For continuous rate monitoring, the requirement to apply water or gel frequently is not acceptable. In U.S. Pat. Nos. 6,371,920 B1 and 6,394,960 B1 attempts were made to overcome this problem by using an array of small transducers protruding from the support surface to make firm contact with a living subject with no air gap in between. However, this increases the complexity and cost of the transducer device and its driving electronics significantly. The air gap will not be totally removed, either, due to body hairs and the variable condition of skin from person to person. In U.S. Pat. No. 6,447,456 B1, two sets of transducers are used at the radial artery and the ulnar artery. The idea is to cope with the compromised signal quality due to motion at the wrist that may create an air gap from time to time. With two sets of transducers the hope is that at least one of them will reliably detect the Doppler signal to identify the heart beat. The disadvantages of continuous mode over pulsed mode are higher cost and more power consumption.